In the options-display menu there is a setting for translucent Copper. For some reasons enabling this option and/or changing the transparency has no effect. The selected layer stays visible and the layer under it is invisible.
When it says copper it means copper pour or template pours, not tracks and pads, like when pulsonix mentions copper in the spacing rules. It shows added copper or poured templates with transparency.
Chris, if you wish to make the Tracks semi-transparent, try using the Merge Colours (Options, Display) . You will need to switch off the Enhanced Graphics option (Options, Display) if you wish to use the Merge Colours option at present.